The crystal structure of perhamite, (Ca,Sr)3Al7.7Si3P4O23.5(OH)14.1.8H2O, from the Emmons mine, Maine, USA, has been determined using single crystal X-ray data. The average structure has trigonal symmetry, P3̅m1, with cell parameters a = 7.021(1) Å and c = 20.218(1) Å. It was refined to R1 = 0.044 for 618 observed reflections. The structure comprises ordered blocks of crandallite-type structure, centred at z = 0, intergrown parallel to (001) with disordered aluminosilicate structure blocks centred at z = 1/2 to form a microporous structure containing large channels along . These channels are bounded by 8-member rings of 6 tetrahedra (2 SiO4, 2 AlO4 and 2 PO4) and 2 AlO6 octahedra. Calcium atoms and water molecules are distributed in the  channels. A model was developed for the local ordering of silicon into the fractionally occupied sites in the (001) layer at z = 1/2 and this model was refined in space group P321 to R1 = 0.041 for 933 observed reflections. The dominant contributors to the local order are Si3O9 rings of corner-shared tetrahedra, together with Si2O7 pairs of tetrahedra. These units corner-link to (Al,Si)O4 tetrahedra above and below the plane at z = 1/2 to form 4-member rings, which in turn corner-share to PO4 tetrahedra in the crandallite blocks to give the 8-member rings. The analysis suggests that 5-coordinated Si may also be present in the (001) plane at z = 1/2.